Before reaching its operating temperature, significant emissions may bypass a catalyst positioned in an engine exhaust. Thus, a common strategy for improving cold start emissions is to increase the heat flux through the engine in order to bring the catalyst up to operating temperature. However, the exhaust system may lose heat between the engine and catalyst, as the length of travel, surface area, number and shape of bends, heat flux, heat transfer coefficients, etc., between the combustion chamber and the catalyst face may pose significant opportunities for heat loss. A turbocharger added along this pathway will absorb a significant and measureable amount of heat that may also delay the catalyst light off time. The turbocharger may also have a negative impact on cold start fuel economy as more air and fuel are needed to overcome this additional heat loss.
One solution for minimizing heat loss via the turbocharger is to divert some or all of the exhaust around the turbocharger and straight to the catalyst, for example by opening a wastegate associated with a turbine positioned in the exhaust. However, wastegates are typically sized to provide optimal flow control for regulating the amount of boost provided by the turbocharger, and even if fully opened, may not provide a high enough level of diversion to rapidly heat a catalyst.
The inventors herein have recognized the above issues and provide an approach to at least partly address them. In one embodiment, a method of controlling a turbocharger of an engine via a multi-staged wastegate includes during a first condition, actuating a first stage of the wastegate based on boost pressure generated by the turbocharger, and actuating a second stage of the wastegate based on a temperature of a catalyst downstream of the turbocharger.
In this way, both boost control and rapid catalyst heating may be provided by a common wastegate. In one example, the wastegate may include a first stage arranged on a second stage. The first stage may be opened based on a desired boost pressure. The second stage may be kept closed while the first stage is open, and may include an opening that is regulated by the first stage. The second stage may be opened when maximal turbine bypass is desired, such as when catalyst temperature is below light-off temperature. By doing so, a relatively large amount of exhaust may bypass the turbine to rapidly heat the catalyst while still allowing for precise control of boost pressure, without the provision of additional bypass lines.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.